The present invention relates to an acoustic rheometer enabling the measurement of the viscoelastic properties of a fluid subjected to periodic shear forces, and a device for the measuring of these properties through the use of the rheometer, notably in the high frequency range.
The value of high frequency measurements of viscosity and elasticity of fluids is that they provide access to the structure of these fluids in relation to their composition. The determining of the mechanical properties has major uses in industrial applications. Indeed these fluids come into play in numerous applications such as the manufacture of printing inks and paints, the agricultural processing industry, the lubrication of mechanical systems, or again the recovery of petroleum products.
In the especially promising application of the rheometer to the manufacturing of inks, since the frequency of the formation of the drops ranges from some tens of kilohertz to about 125 kilohertz, the rheometer should function in oscillating mode at a typical frequency of 100 kilohertz. To this end, the working of the rheometer is based on the following principle: the fluid to be studied is exposed to contact with a single moving surface, propelled to a torsional wave, which creates a simple shearing plane wave within this fluid, at a given frequency located in the 50 to 500 KHz range. The influence of the fluid on this shear wave, reflecting its viscoelastic properties, is measured by means of a variation in the characteristic impedance of the surface. The device used to measure these viscoelastic characteristics of the fluids, also called a rheometer, therefore comprises a transducer, generally made of a quartz crystal which is rigidly fixed to one end of a sensor. This sensor, which is made out of a cylindrical metal rod having the same diameter as the transducer, receives the vibrations from the quartz in the form of a wave train and its lateral surface in contact with the fluid creates a motion of shearing of the fluid. Two quantities characterize the propagation of these torsional waves in the rod: the phase speed B and the attenuation of amplitude A per unit of length. The reference state of the sensor is defined, in the absence of fluid, by a zero characteristic impedance Z* of the fluid, this state being non-dispersive. The propagation of the torsional waves that it receives is therefore characterized by a speed B.sub.o and an attenuation a that are inherent.
The presence of a fluid at the lateral interface of the sensor generates a variation of the attenuation of the amplitude .delta.A of the torsional wave as well as a variation of the phase speed .delta.B with respect to the reference state, these variations being proportional to the characteristic impedance Z* of the fluid. Indeed, the wave travels at a greater speed than in the reference state and leads to a phase-shift variation. EQU .delta.B=B-B.sub.o
Furthermore, the torsional wave with a pulsation is propagated by successive reflections on the lateral limit of the cylindrical sensor, each reflection being accompanied by a dissipation of the acoustic energy by the fluid. This prompts an increase in the attenuation A during the propagation of the wave: EQU .delta.A=A-A.sub.o
On the basis of the relationships of dispersal of the two states of the sensor and of the impedance Z*, the basic formula that relates the cause to the effect: ##EQU1## is a linear relationship, where the constant of proportionality is a characteristic of the sensor, it being known that:
.alpha. is the mass in relation to the volume of the sensor, PA1 v is the speed of propagation of the torsional wave in the rod, when there is no fluid, PA1 a is the radius of the rod, PA1 .delta.A is the variation of phase-shift per unit of length, in nepers per meter, PA1 .delta.B is the variation of attenuation of amplitude per unit of length, in radians per meter.
As has been stated here above, the knowledge of the impedance Z* of the fluid gives access to the desired viscoelastic properties, for this impedance Z* is related by definition to the density .alpha..sub.1 of the fluid which takes account of the inertia and to its complex viscosity B* by the following relationship: EQU Z*=(i.OMEGA..alpha..sub.1 .beta.*).sup.1/2 EQU Z=R+iX
So that the measurements of the variation of amplitude .delta.A and of phase shift .delta.B caused by the fluid in contact with the lateral surface of the cylindrical sensor make it possible to know the real part R and imaginary part X of the impedance Z* and to then obtain the complex viscosity B* of the fluid, given that: EQU .beta.*=.beta.'-i.beta.'' EQU .beta.'=2RX/.OMEGA..alpha..sub.1 EQU .beta.''=(R.sup.2 -X.sup.2)/.OMEGA..alpha..sub.1
The essential element of the high frequency rheometer is the torsional-mode transducer, generally constituted by piezoelectric quartz crystal in the shape of a cylinder, the torsional axis of which is parallel to one of its three second order crystallographic axes (the X axis), the quartz forming part of the class 32 of the triclinical system of the crystals. A torsional deformation is produced by exciting the crystal by an electrical signal by means of two pairs of electrodes oriented by +/-45.degree. with respect to the Y axis, perpendicularly to the X axis. Quartz possesses a very great stability of response in frequency so that the phase-shift introduced by the fluid can be measured with high precision.
However, a quartz rheometer such as this has a first drawback due to the fact that the electrically excited quartz generates a mode of torsion that is not pure but coupled to a radial vibration giving rise to a variation of the diameter of the crystal. This introduces a measurement error, probably due to the surface tension of the fluid, that exceeds the limits of precision hoped for from the stability in frequency. The error in measurement is all the greater as the fluid to be measured has low viscosity: imprecision of + or -15% is possible in the measurements.
A second drawback of a quartz rheometer arises out of the coupling coefficient of quartz, namely the output, in terms of mechanical energy, of the electrical stress applied to the quartz. This output is low, of the order of 3%, and makes it necessary to use excitation voltages of several hundreds of volts so that the sensor, which is coupled to the quartz crystal, receives an acoustic wave that is not greatly attenuated. The effect of the low coupling coefficient is to reduce the signal-to-noise ratio.
Other types of torsional-mode transducers are described in the French patent No. 2 327 677 and in the U.S. Pat. No. 3,719,907. In both these cases, the method used to make the torsional-mode transducer is lengthy, comprising several steps for the metallization of electrodes, polarization with intense electrical fields (several thousands of volts/mm) and selective etching. These different steps have the consequence of increasing the price of the transducer without any improvement, as compared with quartz transducers, in the performance characteristics relating to pure torsion deformation and high coupling coefficient.
Thus, in order to avoid these above-mentioned drawbacks with respect to the complexity of making the transducers, operation at a single frequency and imprecise measurements, above all at low viscosity, the aim of the present invention is the making of a high precision rheometer enabling the measurement of the viscoelastic characteristics of a large variety of liquids, in a wide range of temperatures and pressures. The rheometer according to the invention is furthermore provided with a transducer generating a pure torsion mode, with a high coupling coefficient, great reliability and simplicity of use, while at the same time permitting measurements at several resonance frequencies. This rheometer furthermore comprises a sensor that is is matched in impedance with the transducer and is stable in temperature.